Cathode assembly



Nov. 24, 1959 G. R. MAHN CATHODE ASSEMBLY Filed Jan. 2. 1958 2 Sheets-Sheet 1 INVENTORI- GEORGE R. MAH N ATTORNEY Nov. 24, 1959 G. R. MAHN 2,914,692

CATHODE ASSEMBLY Filed Jan. 2, 1958 2 Sheets-Sheet 2 2ND ACCELERATOR VOLTAGE FIG. 6

IST ACCELERATOR VOLTAGE ANODE CURRENT SHIELD emu m VOLTAGE SHIELD cam CURRENT m ABSENCE OF BLANKING SIGNAL (0) J a CONTROL GRID VOLTAGE CONTROL GRID VOLTAGE WHEN BLANKING SIGNAL IS APPLIED DURING NON- conoucTme PERIOD INVENTOR.--- v GEORGE R. MAHN Waw ATTORNEY CATHODE ASSEMBLY GeorgeRvMalin, Hales Corners, Wis., assignor to General Electric Company, a corporation of New York Application January 2, 1958, Serial No. 706,811

8 Claims. (Cl. 313-82) The present invention relates in general to the control of electron movement in electron flow'devices, and has more particular reference to an improved cathode assembly embodying control means of novel character for obtaining an energy spectrum of desired wave shapein a electron beam emitted from the cathode.

An important object of the present invention is to provide a cathode structure forming an electron gun capable of producing high density electron beams having well defined cross sectional configuration.

' Another important object of the present invention is to provide a cathode structure capable of emitting electrons during sharply defined emission intervals during operation of the cathode under alternating energy excitation, to thereby obtain desirable square configuration in theemitted electron beam.

, Another important object of the invention is to provide a cathode structure embodying means for controlling the sectional size of the emittedelectron beam and the angle of convergence ordivergence of its constituent electrons. Another important object is to provide an electron emitting cathode having a beam forming element which,

in conjunction with an adjacent accelerating electrode, establishes a potential field distribution of character producing substantially constant beam size and shape, despite variations in voltage applied to the accelerating electrode. Another important object is to provide a cathode of the character mentioned in which control and shield voltages applied in the beam shaping components of the cath-- ode'j' are relatively low, thereby permittingcomponents of small size to be employed.v

Another important object is to provide a cathode structure for producing high density electron beams by main taining a suflicient voltage gradient at the emitting 'sur-' face of the cathode whereby the same may operate as an emission limiting cathode; a further object being to provide a cathode structure having a longemitter life and also having high electronic efficiency.

: Another important'object is to provide a cathode structure capable of producing a convergent beam having a small diameter at the anode of an X-ray tube in which the cathode structure may be assembled, or at the emission window of an electron beam generator. Another important object is to provide a cathode struc ture capable of operation to completely out ofi electron emission during any desirable portion or all of the operating cycle of the device.

Another important object is to provide an electron emitting cathode structure which effectively shields the. emitter from variations in accelerating voltages, 'whether' such be normal variations in theenergizing wave, or mere transient changes, changes in phase or changes 1n wave shape due to extraction of high beam currents from the Patented Nov. 24, 1959 ice through a cathode structure embodying the present invention; Fig. 3 is a sectional view taken substantially along the line 33 in Fig. 2;

Fig. 4 is a sectional view taken substantially along the line 44 in Fig. 3;

Fig. 5 is a bottom view of the cathode structure shown in Fig. 4;

Fig. 6 is a graphical representation of voltage and current relationships encountered in the cathode structure of the present invention when the same is in operation as an electron emitter; and

Figs. 7 and 8 are graphical representations respectively of current and energy spectra encountered during the operation of conventional cathodes and the cathode structure of the present invention.

To illustrate the invention the drawings show an electron emitting cathode structure particularly Well suited for use as a cathode component in electron tubes, es-

pecially high voltage multi-section tubes embodying cascade electron accelerating electrodes, of the sort used 9 velope containing or embodying an anode and accelerat- V Westendorp in'High Voltage Apparatus.

ing electrodes disposed in alinement between the anode and cathode, as shown, for example, in U.S. Letters Patent No. 2,144,518 on the invention of Willem F.

Briefly stated, the cathode structure of the present invention comprises. a thermionic electron emitter 12 of any suitable or preferred filamentary or surface type which may be shaped as desired to influence the adjacent potential field. The emitter is preferably mounted close to but insulated from a control element which may comprise a metaldisc 13 forming a head provided with an aperture or opening 14, the emitter 12 being preferably disposed in the aperture 14. The disc 13 carries a control grid 15 comprising an open wire mesh electrically connected with the head formed by the disc 13 and disposed in the opening 14 immediately in front of the emitter 12. r

The cathode structure 11 also embodies a block of electrical conducting material forming an element 16 configurated to control the shape of the adjacent potential field, said element preferably comprising a cylindrical block of metal having a rear surface spaced from and facing the front side of the aperture disc 13. The element 16 is formed with a cup-shaped cavity 17 opening upon the surface thereof which faces away from the disc 13, the element 16 being also formed with an aperture 18 communicating with the bottom of the cavity 17 and opening upon the bottom or disc facing surface of the element 16. The disc or head 13 and the element 16 areso disposed as to present the apertures 14 and 18 in coaxial alinement. a grid 19 comprising an open wire mesh electrically connected with the element 16 and disposed in the'opening The element 16 also carries 3 18 thereof, the grids 15 and 19 being substantially identical.

The cathode structure 11 is employed in conjunction with a shaped accelerating electrode 26 having a generally cylindrical body portion and disposed in spaced apart coaxially alined relationship with respect to the element 16 and on the side thereof which faces away fromthe apertured disc 13.

As shown more particularly in Figs. 2,3 and 4-.o f the drawings, the cathode structure 1 1- may comprisea base plate 21 (Fig. 4) adapted to be sealed in the envelope in .which the. cathode structure is. mounted for use. The

base plate 21 may be provided with conventional conductor sealsembodying conductors F-l and F2i, G and C (Fig. The apertureddisc or head 13 may be secured in and'electrically connected with an end of. a cylindrical support member 23, of electrical conducting material such as copper, the opposite end of the member 23-being secured to, and electrically connected with the base plate 21, whereby thus electrically to connect the disc 13 withthe base plate. v

The emitter 12 may conveniently comprise a wire filament configurated as a spiral and having opposite ends supported on and electrically connected with support stems 24 mounted in spaced relation in an insulating bracket 25 secured on and extending beneath the disc 13 and within the member 23, the stems 24 being electrically connected respectively with the conductors F-l and F-2. The emitter filament may thus be supported in the opening 14 of the disc 13in position immediately behind the grid- 15, which may. comprise a plurality of refractory metal wires of filamentary character, extending in parallel spaced apart relationship transversely of the opening 14. The spaced apart wires forming the gridextend diametrically of a mounting ring26, the ends of the wires being secured to and electrically connected with the ring 25. The ring 26 in turn may be mounted in a, seat 27 formed in the surface of the disc 13 which faces toward the element 16, the ring 26 being secured in the seat 27 as by means of fastening screws 28. v

The element 16 may be mounted on, insulated from, and supported in spaced relation with respect to the disc 13, as by means of. a plurality of circularly spaced apart fastening screws 29 which extend through insulating collars 31 mounted in the disc 13 and like collars 31 of insulating material mounted in seats formed in the element 16 and disposed in positionto relatively insulate the element 16 from the disc 13. The fastening screws 29 preferably have threaded engagement in'openings 32 formed in the element 16 and are hence electrically connected with said element. The screws, however, extend freely through openings 32 formed through the disc 13, in which openings the insulating elements 30 are seated, whereby the fastening screws 29 are insulated from the disc 13. One of the screws 23 may be electrically connected with the conductor C, as by means of a wire 33, tothereby electrically connect the element 16 and the grid 19 with the conductor C. The conductor G may be electrically connected with'and utilized to energizeconventional getteringmeans within the cylindrical support member 23. The grid U also may comprise a plurality of parallel spaced apart refractory metal wires of filamentary character supported on. and extendingdiametrally of a mounting ring 26, which inturn may be secured in a seat 27, formed in the side of the element 16 which faces toward the. disc 13, as by fastening screws 28. Y

It will be seen from the foregoing that the control grid 15 may be electrically connected through the disc 13, the mounting sleeve 23, and the base plate 21- with a connection stud S on the base plate The stud S may be electrically connected with the negative side of a uni-directional high voltage potential source,.the. positive.

side of which is connected withthe conductor. Felland hence with the emission, element 12. Any suitable source of energizing power may be connected between the conductors F-l and F-2 for the purpose of energizing the emission element. The shield grid 19 may be electrically connected, through the element 16, the stud 29, the connecting wire 33 and the conductor C, with the relatively positive side of a high voltage uni'directional potential source, the relatively negative. side of said source being connected with the conductor F-l and hence with the emission element 12'.

The cathode structure of the. present invention is preferably operated at a relatively high negative voltage with respect to ground of the order of 250 kv. to 4 mev., and it may be connected to derive all power for filament excitation, for control and for. shield grid power, and for accelerator electrode power from cascading windings located at the extremity of a high voltage resonant type transformer. In the interests of minimum size and weight, the transformer and tube head structures containing the cathode assembly of the present invention may be of compact design affording very little space for con trol and power supply components in the cathode region.

Because of high voltage and space considerations, it is.

also difiicult to translate motions. and electrical and magnetic signals from ground potential control elements to the cathode. As a consequence, the electron source and its associated power supply components become static elements and have to beself compensating to a large degreeinresponse to changes, such as voltage level, voltage regulation due. to load, phase angle, wave shape distortions, transients and other operational phenomena, that take place in the resonant transformer when in operation. Furthermore, the operation of the device can be measured only indirectly, as by observation of anode current and accelerating voltage oscillograms- When inoperation, the emission element 12 is preferably caused to function under steady state conditions, that is to'say, the. temperature of the emission element, which is adjusted by means of a variable reactor in the energizing circuit and which determines electron emission, is held constant at a selected level in order to cause emission at: desired intensity as theprirnary limitation of anode current. In this connection, it should be understood. that anode current is the current which passes through. the final: accelerating electrode of an electron flow device containing the cathode structure of the present invention, anode current, of course, being a function. of electron emission from the cathode structure. Such emission limited operation of the electron emission element eliminates the necessity of providing amplitude control of control. grid voltage. It is, however, contemplated. that the cathode couldbe operated as a spacecharge-limited device.

The control grids. 15 and 19may be made of refractory metal wires, such as molybdenum, or tungsten, and preferably have a transparency-of about or higher. As shown. more particularly-in Fig. 6 of the drawings, a pulsating. signal may be applied to the control. grid 15 to control anode currentfrom cut-off to a maximum over any portion of the accelerating voltage cycle desired. The signal .voltage thus applied to the control grid is essentially a series of square wave pulses, which may befrom to cycle wide, centered about the peak portions. of. the accelerating voltage wave.

The wires of the shield and control gn'dsbeing preferably incolinear registration,. a-positive voltage relative to the emitter 12 may be applied to the shield grid, or 1t maybe energized by alternating energy in phase with anode-cathode. potential alternations.- The shield grid functions to..establisha high positive voltage gradient at the emitter surface during'the conducting period of the device in order to draw off maximum, anode. current. It also shields the emitter to a-large extent from the effect offluctuating voltage applied to the accelerator electrode 20 which is. energized directly frornthe resonant transformer. Since the wires of the shield and control grids are colinear, the control grid effectively prevents electrons from reaching the shield grid and lowering the efliciency of the electron source. The high transparency of the grid structures, however, enables the positive potential field to extend well in toward the emitter, thereby promoting electron emission.

- The shield grid electrode 16 and the accelerating electrode 2 0 are designed to produce a convergent electron beam, sothat only a small fractional portion of the total current is collected by the accelerating electrode. Furthermore, the well collimated beam produced by the cathode structure of the present invention is relatively easy to control after it passes the accelerating electrode 20 by means of simple two-element cylindrical electron lenses arranged in cascade. Electron flow tubes embodying the cathode structure of the present invention, when excited by electrical energy of the sort delivered through aresonant transformer, avoid the eifects of varying potential fields in the cathode region which undesirably afiect tube current in devices containing cathodes of conventional character. Tubes containing the cathode structure of the present invention are well adapted to produce high density and sharply defined electron beams having small cross sectional area, beams produced in devices embodying conventional cathode structures being lacking in one or more of such desirable characteristics. The present invention, furthermore, provides for obtaining maximum electronic efficiency in electron beam generating tubes, the same being highly desirable because of further limitations in available heater power, heat dissipation, and life expectancy of the emitter. V

The cathode structure of the present invention, moreover, is capable of producing a sharply controlled conduction period during operation, the same being highly desirable in order to produce beams in which the electron energy range is as nearly uniform as possible, to thereby obtain uniform penetration of electrons in material upon which the beams may be applied. Idealized pictures of current, voltage and energy relationships, in electron beam tubes containing cathode structures embodying the present invention, are shown in Figs. 7 and 8, in which the curve 41 designates a sine wave voltage distribution, curve 42 illustrates the anode current obtained during the operation of a cathode structure embodying the presentinvention, while curve 43 illustrates the anode current obtained in electron beam tubes embodying conventional cathode structures. In Fig. 8, the curve 44 illustrates the energy spectrum of an electron beam produced in a generating tube embodying a cathode structure of the present invention, while curve 45 illustrates the energy spectrum of an electron beam produced in a tube embodying a conventional cathode Structure, the cross-hatched portions showing the unwanted, low energy components of beams produced by use of conventional cathodes.

-The present invention furthermore provides for desirable control of electron beam diameter and angle of convergence or divergence, the same being factors of considerable importance in the design of both X-ray and electron tubes. In X-ray tubes it is necessary to maintain focal spot sizes at high current densities in order to obtain maximum X-radiation, the focal spot being the area of electron impact upon the target surface of the X-ray tube anode. In electron beam generators, it is desirable to control the beam so that its diameter is a minimum at the electron beam emission window of the tube, the beam being ordinarily deflected longitudinally and transversely of the window in order to cover all or any desired portion of the emission window. The cathode structure of the present invention includes a beam forming element 16, which, in conjunction with the next adjacent accelerating electrode 20, establishes a potential field distribution'which determines beam size and direction. The cathode structure, moreover, is adapted to produce beams having desired size and shape which remain constant despite variation in voltage applied to the accelerating electrode.

as well as in the associated tube enclosure, it is desirable to keep the control and shield voltages as low as possible, in order to minimize component size, which is generally dictated by insulation requirements. Potentials of the order of 1000 volts are employed in the cathode construction of the present invention, such potentials being easily supplied and insulated.

Cathode structures embodying the present invention produce high density electron beams by maintaining an adequate voltage gradient at the emitter to cause same to operate as an emission limited cathode. Currents of several hundred 'milliamperes, peak, are easily obtainable. The cathode of the present invention, moreover, has high electronic efiiciency, produces a sharply convergent beam having a small diameterat the anode or emission window, and has operating characteristics adapted to promote long emitter life. In operation, the cathode structure is capable of completely cutting off anode current during any desirable portion or all of the energizingcycle;and the structure is adapted eifect-ively to shield the emitter from variations in accelerating voltages whether the same be normal variations in the energizing wave, or mere transient changes, changes in phase, or in wave shape due to extraction of high beam currents from the transformer. The cathode, furthermore, is capable of producing electron beam wave shapes of controlled form which may duplicate the wave shape of the voltage applied to the control grid.

The device is adapted to operate at relatively low voltage levels, potential of the order of 1200 peak negative volts being employed on the control grid 15, while the shield grid 19 may be energized at a positive voltage of the order of 1000 volts or in phase A.C. volts. Current collected by the shield grid during the conduction interval is negligible even through the shield grid is maintained positive relative to the emitter. During nonconducting intervals the current collected by the shield grid is also negligible, provided suitable values of series resistance is placed in its circuit. Current collected by the shield grid during the non-conducting interval is negligible if the positive voltage relative to the emitter is reduced below a few hundred volts.

It is thought that the invention and its numerous attendant advantages will be fully understood from the foregoing description, and it is obvious that numerous changes may be made in the form, construction and arrangement of the several parts Without departing from the spirit or scope of the invention, or sacrificing any of its attendant advantages, the form herein disclosed being a preferred embodiment for the purpose of demonstrating the invention.

The invention is hereby claimed as follows:

1. A cathode structure comprising a head provided with an opening, an electron emitting element insulated from said head and supported in position alined with said opening, a control grid disposed in said opening in front of said emitting element and being electrically connected with said head, a block of electrical conducting material having a rear surface spaced from and facing said head, said block being formed with a cup-shaped cavity facing away from said head and an aperture at the bottom of said cavity opening upon the bottom of said block in coaxial alinement with the opening in the head, ashield grid disposed in said aperture and being electrically connected with said block, means to electrically energize the emitting element for electron emission, and means to electrically connect said grids respectively with different sources of biasing power.

2. A cathode structure as set forth in claim 1, wherein said grids have a transparency of the order of 3. A cathode structure as set forth in claim 1, where Because. of limited space available in electron and X-ray tubes,

V wherein the electron emitting element comprises a spirally configurated wire of filamentary character mounted within the opening of'the head and insulated therefrom, anddisposed in position immediately behind the head mounted grid.

5. A cathode structure as set forth in claim 1, wherein the'block is supported on'and insulated from the head, and is electrically connected with fastening members extending through openings formed in the head and insulated therefrom.

6;; A cathode structure as set forth in claim 1, and including a shaped accelerating electrode insulated from anddisposed in front of and in coaxial alinement with respect to said block;

7. A cathode structure as set forth in claim 1, wherein a said grid comprises parallel spaced apartwires of filamentary character and a mounting ring upon-which said wires are secured, said wires extending diametrally of the mounting ring and being electrically connected at their opposite ends on said ring, and means to detachably secure the ring on and in electrical contact with a cathode structure component.

' 8* a 8. In' an electron flow device, the combination, with an anode element, of a cathode structure mounted in spaced relation with respect to the anode element and comprising a head provided with an opening, an electron emitting element insulated from said'head'and sup ported in position alined with said opening,,acontrol griddisposed in said opening in front of said emitting element and being electrically connected With'said head, a block of electrical conducting material having a rear surfacespacedfrom and facing said head, said block being formed With a cup-shaped cavity facing away-from said head and an aperture at the bottom of said'cavity opening upon the bottom of said block in coaxial alinement with the opening in the head, a shield grid disposed in said aperture and being electrically connected with said block, means to electrically energize the-emitting element for electron emission, and means to electrically connect said grids respectively with different sources of biasing power.

References Cited in the file of'this patent UNITED STATES PATENTS 2,260,927 Atlee Oct. 28, 1941 2,316,214 -Atl'ee Apr. 13, 1943 2,825,837 Dudley Mar. 4, 1958 

